Typically, semiconductor chips are tested to verify that they function appropriately and reliably. This is often done when the semiconductor chips are still in wafer form, that is, before they are diced from the wafer and packaged. This allows the simultaneous testing of many semiconductor chips in parallel, creating considerable advantages in cost and process time compared to testing individual chips once they are packaged. If chips are found to be defective, they may be discarded when the chips are diced from the wafer, and only the reliable chips need be packaged. Semiconductor chips may also be tested after dicing, but before packaging, by assembling die on tape or a mechanical carrier.
Generally, modern microfabricated (termed MEMS) microelectronic contactor assemblies, including probe card assemblies for testing semiconductors, have at least three components: a printed circuit board (PCB), a substrate to which thousands of microelectronic contactors are coupled (which substrate may be referred to as the “probe contactor substrate”), and a compressible electrical interconnect (often in the form of an electrical “interposer”). The compressible electrical interconnect electrically connects the individual electrical contacts of the PCB to corresponding electrical contacts on the probe contactor substrate, which probe contactor substrate then relays signals to and from individual microelectronic contactors. The combination of the probe contactor substrate and its microelectronic contactors is sometimes referred to as a probe head.
The microelectronic contactors on the probe contactor substrate often have a very fine pitch (i.e., small distances between contactors, such as 30 μm to 200 μm) while the electrical contacts of the PCB and the interposer often have coarser pitches (>200 μm). Thus, in modern MEMS probe card assemblies, the probe contactor substrate often provides a space transformation of electrical contracts as it connects the finely pitched microelectronic contactors to the coarser pitched electrical contacts found on the interposer and PCB. Alternately, part or all of this space transformation may be off-loaded to a separate space transformer substrate of the probe head, or to other substrates or components. It is noted that some probe card assemblies do not utilize an interposer, but the general idea is unchanged.
In most applications, the required number of interconnects that need to be made between the substrates of a probe card assembly are in the thousands or tens of thousands, dictating that the PCB and the probe head be parallel (or very close to parallel), and in close proximity, so that the many interconnects therebetween can be reliably made. It is also noted that the vertical space between the PCB and the probe contactor substrate is generally constrained to only a few millimeters.
So that reliable connections to a wafer or other array of semiconductor devices can also be made, it is also important that the tips of the microelectronic contactors on the probe head lie essentially in a plane. The background of U.S. Pat. No. 7,180,316, titled “Probe Head with Machined Mounting Pads and Method of Forming Same,” assigned to Touchdown Technologies, Inc. of Baldwin Park, Calif., discusses the importance of the planarity of the microelectronic contactor tips (or probe tips).
Reliable connections to the pads on a semiconductor wafer become evermore difficult as semiconductor dies and their pads continue to shrink in size and pitch, and as the number of electrical contacts between a probe head and a wafer under test increases (e.g., as the semiconductor test industry enters the era of one touchdown full wafer test). Currently, the bond pads on a wafer can be as small as 30 μm square, but are more typically 50 μm square. The contacts to the bond pads must stay well within the boundaries of the bond pads, and preferably near their centers, as any contact with the peripheries of the bond pads can damage the pads and cause yield loss. Because the bond pads are distributed across the entire surface of a wafer (e.g., across a 300 mm diameter array), precision alignment of microelectronic contactors and bond pads is becoming more and more critical. An approach which binds all of the microelectronic contactors to a single plane, without any need for mechanical adjustment, would be preferred.
It is noted that, in the following description, like reference numbers appearing in different drawing figures refer to like elements/features. Often, therefore, like elements/features that appear in different drawing figures will not be described in detail with respect to each of the drawing figures.